1. Field of the Invention
The present invention relates to a direct sequence spread spectrum communication system, and, more particularly, to an automatic gain control process and controller for automatically controlling a gain of a received signal.
2. Description of the Related Art
A direct sequence spread spectrum communication system transmits a signal by a packet unit and needs to maintain an optimal reception level during a received packet interval to minimize an interference caused in a modem at a base band. In particular, in case of a LAN (Local Area Network) used for a local communication, a communication system may receive an input signal whose intensity exceeds a maximum permissible reception level. Thus, it is necessary for the communication system to include a reception gain control circuit for appropriately controlling the high power input. Such an automatic gain control circuit is disclosed in U.S. Pat. No. 5,233,634 to Vaisanen, issued on the 3.sup.rd of August, 1993, and entitled Automatic Gain Control Circuit In A Radio Telephone Receiver.
In these designs for direct sequence spread spectrum communication systems, most of the total gain control of the received signal is accomplished in the base band frequency sections, i.e. in the I and Q branches, where the signal amplification is controlled step by step, and only a minor part of the total gain control is performed in the radio frequency (RF) stage.
High amplification steps are formed by amplifiers, wherein the desired amplifier is activated by digitally controlled multiplexers, and low amplification steps are formed by resistance attenuators, in which a signal of a desired level is selected by digitally controlled multiplexers. In addition, an RF amplifier has two amplification levels and the desired amplification is selected by a digital control. With the different digital control combinations, the entire gain control range required of the receiver can be covered.
Exemplary designs suggested for contemporary circuits include those found in U.S. Pat. No. 5,168,505 to Akazawa et al., entitled Automatic Gain Control Device For Spread Spectrum Communication Device, U.S. Pat. No. 5,566,201 to Ostman, entitled Digital AGC For A CDMA Radiotelephone, U.S. Pat. No. 5,644,590 to Sugita, entitled Spread spectrum Communication Apparatus And Signal Intensity Detection Apparatus, U.S. Pat. No. 5,347,534 to Akazawa et al., entitled Automatic Gain Control System, U.S. Pat. No. 4,899,364 to Akazawa et al., entitled Automatic Gain Control System, U.S. Pat. No. 4,953,178 to Ishigaki, entitled Spread Spectrum Communication System, U.S. Pat. No. 5,631,921 to Schilling, entitled Adaptive Power Control For A Spread Spectrum Communications System And Method, U.S. Pat. No. 5,535,238 to Schilling et al., entitled Spread Spectrum Adaptive Power Control Communications System And Method, U.S. Pat. No. 5,548,616 to Mucke et al., entitled Spread Spectrum Radiotelephone Having Adaptive Transmitter Gain Control, U.S. Pat. No. 5,276,685 to Kepler et al., entitled Digital Automatic Gain Control, U.S. Pat. No. 5,481,226 to Parkes Jr., entitled Low-Voltage Intermediate Frequency Amplifier Providing Automatic Gain Control Of A Source Amplifier, U.S. Pat. No. 5,630,220 to Yano, entitled Automatic Gain Control Circuit For Radio Communication Apparatus, U.S. Pat. No. 5,509,030 to Mortensen, entitled RF Receiver AGC Incorporating Time Domain Equalizer Circuitry, U.S. Pat. No. 5,507,023 to Suganuma et al., entitled Receiver With An AGC Circuit Capable Of Expanding A Dynamic Range, U.S. Pat. No. 5,493,712 to Ramesh et al., entitled Fast AGC For TDMA Radio Systems, and U.S. Pat. No. 5,438,683 to Automatic Level Control Circuit For Dual Mode Analog/Digital Cellular Telephone.
I have found however, the contemporary receiver designs need a controller in order to calculate very complex numerical formulae for a digital control of the signals, and a large number of serially-coupled amplifiers each having different gains for controlling the gain. Besides, the conventional automatic gain controller includes a plurality of the multiplexers and resistors with fixed resistances. Since the prior art device uses a large number of elements, the cost may increase undesirably and it may be difficult to realize a compact device. Further, since the amplifiers and the resistors have fixed gains and resistances, it may be difficult to finely control the gain.